The present disclosure relates to pneumatic expander brake/clutch devices employed for braking or clutching power transmission on rotating elements and machinery such as cable drums and marine propeller shafts, where it is desired to brake or engage power transmission between shafts or between a shaft or drum and a stationary element. Such devices are often employed to transmit or dissipate substantial amounts of shaft power, for example, brakes or clutches capable of handling torque of 300,000 newton meters (N.m) and having diameters up to 77 inches (193 cm) for applications such as, for example, grinding mills, marine propulsion, metal forming machinery and oil field machinery. Such relatively large pneumatic brakes/clutches may employ a drum having a peripheral frictionally engaging surface against which are forced friction pads on an array of arcuately configured circumferentially disposed backing blocks which force the friction material or friction pads thereon against the drum friction surface under the action of an inflatable annular ring or expander surrounding the array of arcuate backing blocks.
In service, such relatively large brakes/clutches generate heat upon the drum friction surface during braking/clutching; and, the heat radiates outwardly in the space between the arcuate backing blocks disposed about the drum. Heretofore, in service with such large brakes/clutches, it has been found that when the temperature on the friction surface of the drum reaches about 600° F. (315° C.) the temperature of the inflatable surrounding the arcuate blocks can exceed the 250° F. (121° C.) service limiting temperature for the materials, typically rubber, employed in the inflatable ring. This temperature limitation of the inflatable ring thus limits the permissible temperature rise on the friction surface of the drum and, thus, the amount of power transmitted or dissipated by the brake/clutch assembly.
Thus, it has been desired to provide a way or means of increasing the allowable surface temperature of the friction surface on the drum of a pneumatically operated brake/clutch assembly in order to increase the amount of power or torque transmitted or dissipated by the brake/clutch assembly for a given size drum friction surface without overheating the expander ring.